Absorption by a solvent circulating in a loop is a method which is commonly used when a constituent or a certain category of constituents has to be at least partially extracted from a stream of gas.
The method used generally comprises two stages, a stage of absorption proper, when the gas to be treated and the solvent are put into contact in reverse directions in a vertical column, then a stage of solvent regeneration, when the gas fraction absorbed during the first stage is desorbed by the action of heat or pressure or both. The solvent thus regenerated can then be recycled to the first stage.
In cases where the solvent temperature has to be raised in order to regenerate the solvent, it is essential to recover the heat applied, in order to obtain a good heat balance for the process. The means generally employed is described below, with reference to FIG. 1.
The solvent leaving the absorption column C1 through the pipe 4, after possible expansion in the valve V1, and the regenerated solvent leaving the regenerating column C2 through the pipe 8, via the pump P1 and pipe 9, pass into a heat exchange zone E1. On leaving that zone the charged solvent returns to the regenerating column C2 through the pipe 6 in a hot, partially vaporized state, thereby saving energy in the reboiler of the regenerating column. The cold regenerated solvent is passed to the absorption stage through the pipe 10, since absorption is generally encouraged at low temperature. The heat supplied to the reboiler of column C2 is thus partially recovered in the heat exchanger E1. However, this process causes irreversible heat losses, owing to the fact that the solvent vaporized in the heat exchanger E1 and the reboiler of column C2 has to be recondensed at the top of column C2 by a cold fluid external to the process.